Optical information memory medium recording and erasing information including gallium and antimony

ABSTRACT

Recording and erasing optical information can be done by using an alloy film capable of forming two stable crystalline states differing in crystal texture and optical characteristics by being irradiated with optical energies under different conditions. The thin memory film preferably includes not more than 60 atom % of Gallium (Ga) and not less than 40 atom % of Antimony (Sb).

This application is a continuation of application Ser. No. 07/536,802,filed June 12, 1990, now abandoned, which is a divisional of applicationSer. No. 07/401,499 filed on Aug. 3, 1989, which is a continuation ofSer. No. 101,367 filed Sept. 25, 1987, now abandoned, which is adivisional of Ser. No. 06/803,294, filed Dec. 2, 1985, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to medium for optically storinginformation as in an optical disk. More particularly, the presentinvention relates to an optical information memory medium in whichonce-recorded information can be erased and new information can berecorded. The invention also relates to methods and apparatus forrecording and erasing and reading information by using such an memorymedium.

2. Description of the Related Art

Since a high memory speed and density are enabled in the optical storingof information, the optical information storing method has attractedattention as a promising information storing method. Among theconventional optical information memory mediums, there is a medium inwhich a metal film is irradiated with laser beams and fine holes areformed at the irradiated part to store information. In this medium,recording is possible, but the medium is limitative in that erasure ofthe recorded information and the recording of new information areimpossible. As the memory medium in which not only the optical recordingof information but also erasure, and re-recording are possible, there isknown a memory medium in which a film of an amorphous semiconductor suchas Te₈₁ Ge₁₅ S₂ P₂ is used, and two structural states, that is, a stablehigh-resistance state (the so-called amorphous state where thearrangement of atoms or molecules is disturbed) and a stablelow-resistance state (the so-called crystalline state where atoms ormolecules are regularly arranged), are reversibly interchanged to effectthe recording, erasure, and re-recording of information (see U.S. Pat.No. 3,530,441, and Japanese Examined Patent Publication (Kokoku) No.47-26897).

However, in the above-mentioned erasable memory medium, since the stateof the disturbed atom arrangement (amorphous state) is used as onestate, retention of the information is inherently unstable. This isbecause the amorphous state is a metastable state leading to thecrystalline state, and the amorphous state is readily changed to thecrystalline state by the application of thermal energy or chemicalenergy, and thus stored information is easily lost. Moreover, sincetransition is effected between two greatly different states, that is,the amorphous and crystalline states, fatigue occurs during the repeatedrecording and erasure, and accordingly, the number of repetitions ofrecording and erasure is limited.

Investigations have been made into alloys which are similar to thesubject materials of the present invention, but these materials have notbeen used as a memory medium in which information is recorded and erasedbetween two stable crystalline states, since the above investigationswere directed to finding a memory medium capable of assuming two states,i.e., between the amorphous and crystalline states (for example, M.Wihl, M. Cardona and J. Tauc, "RAMAN SCATTERING IN AMORPHOUS Ge andIII-V COMPOUNDS, Journal of Non-Crystalline Solids 8-10 (1972), 172-178;G. Fuxi, S. Baorong and W. Hao, GLASS FORMATION OF SEVERALSEMICONDUCTORS AND ALLOYS BY LASER IRRADIATION, Journal ofNon-Crystalline Solids 56 (1983), 201-206; W. Eckenback, W. Fuhs and J.Stuke, PREPARATION AND ELECTRICAL PROPERTIES OF AMORPHOUS InSb, Journalof Non-Crystalline Solids 5 (1971), 264-275; J. Feinleib, J.deNeufville, S. C. Moss and S. R. Obshinsky, RAPID REVERSIBLELIGHT-INDUCED CRYSTALLIZATION OF AMORPHOUS SEMICONDUCTORS, AppliedPhysics Letters, Vol. 18, No. 6, Mar. 3 (1971) 254-257; IBM Thomas J.Watson Research Center, LASER WRITING AND ERASING ON CHALCOGENIDE FILMS,Journal of Applied Physics, Vol. 43, No. 11, Nov. (1972), 4688-4693).

DISCLOSURE OF THE INVENTION

It is a primary object of the present invention to provide an opticalinformation memory medium in which information is recorded byirradiation with optical pulses, the recorded information can be erasedif required, or the information can be stably retained.

In order to attain this object, in accordance with the presentinvention, a film composed of an aggregate of crystallites having aregular atom arrangement, which includes at least two stable statesdiffering in optical characteristics, is irradiated with two kinds ofoptical pulses differing in power and time width, and one of theabove-mentioned two states is produced to store information. In thepresent invention, both of the stable states of the memory film forrecording information, which differ in optical characteristics, arecrystalline states, and the transition between two crystalline stablestates differing in optical characteristics is utilized. In the presentinvention, in order to distinguish the crystalline state from theamorphous state, the term "crystalline" denotes that the size of theregion of the film having a regular atom arrangement (the particle sizeof the crystallite) is at least about 5 nm and ordinarily 20 to 30 nm orlarger.

Since the two stable states of the crystalline memory film of thepresent invention are reversibly interchanged by irradiation withoptical pulses under appropriate conditions, once-recorded informationcan be erased and the film can be used repeatedly.

Ordinarily, the two stable states of the crystalline film have a highelectric conductivity and there is no substantial difference between theelectric conductivities of the two states (the electric conductivity ofthe amorphous state is essentially lower than that of the crystallinestate).

The two crystalline stable states of the crystalline film are slightlydifferent in optical characteristics such as light reflectance and lighttransmittance, and therefore, the state of recording of information andthe state of erasure of information can be discriminated based on thedifference of reflectance. Moreover, the two stable states accompany aslight change of the volume and a slight deformation of the shape of thefilm, and therefore it is supposed that the optical difference isconsequently increased.

This memory medium does not utilize the transition between the amorphousstate and the crystalline state. Since the amorphous phase is ametastable phase, the amorphous phase is gradually transformed into thecrystalline phase by the action of heat over a long period. Accordingly,if the difference between the two phases is utilized for the storing ofinformation, the information is easily lost. In contrast, in the presentinvention, transition is effected between two states of onethermodynamically stable phase, that is, the crystalline phase, andtherefore, information can be stably retained for a long time.

As the material of the crystalline film showing at least two stablestates differing in crystal structure and optical characteristics, therecan be mentioned, for example, an alloy comprising 80 atom % or less,preferably 15 to 50 atom %, more preferably 35 to 45 atom %, of indium(In) and 20 atom % or more, preferably 50 to 85 atom %, more preferably55 to 65 atom %, of antimony (Sb). Furthermore, an alloy comprising Inin an amount of 80 atom % or less and Sb in an amount of 20 atom % ormore, and additionally comprising at least one element selected fromaluminum (Al), silicon (Si), phosphorus (P), sulfur (S), zinc (Zn),gallium (Ga), germanium (Ge), arsenic (As), selenium (Se), silver (Ag),cadmium (Cd), tin (Sn), tellurium (Te), thallium (Tl), lead (Pb),bismuth (Bi), etc. in an amount of up to 20 atom %, preferably 5 to 20atom %, based on the total elements can be mentioned.

There can be also mentioned an alloy comprising 70 atom % or less,preferably 10 to 40 atom %, more preferably 20 to 30 atom %, of Tl and30 atom % or more, preferably 60 to 90 atom %, more preferably 70 to 80atom %, of Bi. An alloy comprising Tl in an amount of 70 atom % or lessand Bi in an amount of 30 atom % or more, and additionally comprising atleast one element selected from Al, Si, P, S, Zn, Ga, Ge, As, Se, Ag,Cd, In, Sn, Sb, Te, Pb, etc., in an amount of up to 20 atom %,preferably 5 to 20 atom %, based on the total elements also can bementioned.

There can be also mentioned an alloy comprising 85 atom % or less,preferably 30 to 60 atom %, more preferably 40 to 60 atom %, of Ga and15 atom % or more, preferably 40 to 70 atom %, more preferably 40 to 60atom %, of Bi. And an alloy comprising Ga in an amount of 85 atom % orless and Bi in an amount of 15 atom % or more, and additionallycomprising at least one element selected from the group of Al, Si, P, S,Zn, Ge, As, Se, Ag, Cd, In, Sn, Sb, Te, Tl, Pb, etc., in an amount of upto 20 atom %, preferably 5 to 20 atom %, can be mentioned.

There can be further mentioned an alloy comprising up to 65 atom %,preferably up to 50 atom %, more preferably 15 to 25 atom %, of In and35 atom % or more, preferably 50 atom % or more, more preferably 75 to85 atom %, of As. An alloy comprising In in an amount of up to 65 atom %and As in an amount of 35 atom % or more, and additionally comprising atleast one element selected from Al, Si, P, Si, Zn, Ga, Ge, Bi, Se, Ag,Cd, Sn, Sb, Te, Tl, Pb, etc., in an amount of up to 20 atom %,preferably 5 to 20 atom % also can be mentioned.

There can be further mentioned an alloy comprising 60 to 90 atom %,preferably 67 to 90 atom %, more preferably 70 to 75 atom %, of In and10 to 40 atom %, preferably 10 to 33 atom %, more preferably 25 to 30atom %, of Bi. And an alloy comprising In in an amount of 60 to 90 atom% and Bi in an amount of 10 to 40 atom %, and additionally comprising atleast one element selected from Al, Si, P, S, Zn, Ga, Ge, As, Se, Ag,Cd, Sn, Sb, Te, Tl, Pb, etc., in an amount of up to 20 atom %,preferably 5 to 20 atom %, can be mentioned.

There can be still further mentioned an alloy comprising 60 atom % orless, preferably 5 to 50 atom %, more preferably 15 to 35 atom %, of Gaand 40 atom % or more, preferably 50 to 95 atom %, more preferably 65 to85 atom %, of Sb. An alloy comprising Ga in an amount of 60 atom % orless and Sb in an amount of 40 atom % or more of Ga, and additionallycomprising at least one element selected from Al, Si, P, S, Zn, Ge, As,Se, In, Sn, Te, Bi, Pb, etc., in an amount of up to 20 atom %,preferably 5 to 20 atom %, also can be mentioned.

We also found that alloys comprising two or more elements capable offorming an eutectic mixture and having a composition close to aneutectic mixture in an equilibrium diagram of the above two or moreelements, are desirably used as the material of the memory film showingtwo stable crystalline states differing in crystal texture and opticalcharacteristics, or the material of the memory film of the opticalinformation memory medium according to the present invention.

The term "eutectic phenomenon" is well known in metallurgy and denotes aphenomenon wherein a mixture of two or more elements having a certaincomposition shows a lower melting point than the original melting pointsof the consistent element(s) or compound(s). If an alloy having aneutectic composition is solidified from a fused state, the respectiveconstituent element(s) or compound(s) are not mixed with each other inan order of atom or molecule size, but crystallites of the constituentelement(s) or compound(s) are uniformly mixed with each other in asolidified alloy.

Phase diagrams of various alloys, including In-Sb, Tl-Bi, Ga-Bi, In-As,In-Bi, and Ga-Sb systems, have been known and are published, forexample, as "Constitution of binary alloy", from MCGRAW-HILL.

A film of these materials is formed on a glass, plastic or metalsubstrate by effecting alloying by co-vacuum-deposition, co-sputteringor co-ionplating of the starting components. Furthermore, the alloyedmaterial may be vacuum-deposited or sputtered.

In the as-deposited film, the atom arrangement is ordinarily disturbedand the film is amorphous. However, if the film is heated or irradiatedwith light, the entire film or only the recording portion of the film iscrystallized.

Thus, in other aspects of the invention, there are provided methods andapparatus for recording and/or erasing optical information, wherein amemory film is irradiated with optical energies under differentconditions to selectively form two stable crystalline states differingin crystal texture and optical characteristics at the portion of thememory film irradiated by the optical energies, whereby recording and/orerasing information is effected.

In an embodiment of the method and apparatus according to the invention,by irradiating a spot of the memory film with optical energies underdifferent conditions, the spot of the memory film is fused and thensolidified to form different distributions of crystallites havingdifferent reflectances or transmittances within the spot of the memoryfilm such that the reflection or transmission of an optical beam from orthrough the spot of the memory film is different.

In a further embodiment of the method and apparatus according to theinvention, by irradiating a spot of the memory film with opticalenergies under different conditions, the spot of the memory film isfused and then solidified to form two states having compositions at acentral portion of the spot wherein the amount of a certain element orcompound of the memory film is higher and lower than that of an eutecticmixture consisting of the certain element or compound and the otherelement or compound of the memory film, whereby recording and/or erasingis effected.

In further aspects of the invention, there are also provided a methodand apparatus for reading optical information, wherein informationrecorded by selectively forming two stable crystalline states in amemory film are read by optically detecting the selectively formed twostable crystalline states.

According to the present invention, recording can be effected at a highdensity only by irradiating the film with light pulses, and erasure andre-recording can be performed when required. Furthermore, informationcan be stably retained for a long time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the optical system for the storing andreproduction of optical information according to the present invention;

FIG. 2 is a graph showing the change of the reflectance of the memoryfilm according to the conditions of laser light irradiation;

FIG. 3 is a graph showing the relative contrast of recording withrespect to the irradiation time for recording;

FIGS. 4A and 4B are electron microscope photos of the electron beamdiffraction pattern of the low-reflectance portion of the recordingmedium and the crystal structure of the film in the low-reflectanceportion;

FIGS. 5A and 5B are electron microscope photos of the electron beamdiffraction pattern of the high-reflectance portion of the recordingmedium and the crystal structure of the film in the high-reflectanceportion;

FIG. 6 is a typical equilibrium diagram of a simple binary alloy system;

FIGS. 7A and 7B are schematical plan views of the two crystal structuresat the portion irradiated with laser beams under different conditionsrespectively;

FIGS. 8 and 9 are sectional views showing the main part of the opticalinformation memory medium for use in carrying out the present invention;

FIG. 10 is a graph showing the temperature dependency of the electricalconductivity of the InSb film;

FIG. 11 is a graph showing the change of the C/N ratio of the InSb filmwith the lapse of a long period of time;

FIG. 12 is an equilibrium diagram of an InSb alloy system;

FIG. 13 is a ternary phase diagram showing whether or not segregation iscaused when Se is added to the InSb system;

FIG. 14 is a graph showing the reflectance contrast, obtained when As isadded to InSb, relative to the laser pulse power;

FIGS. 15 and 16 are graphs showing the changes of the reflectance andC/N of the TlBi film with the lapse of a long period of time;

FIG. 17 is a graph showing the change of the contrast of the TlBi filmadded with As with the lapse of a long period of time;

FIG. 18 is a graph showing the change of the contrast, obtained when Pbis added to the TlBi film, relative to the laser pulse power;

FIGS. 19 and 20 are graphs showing the changes of the reflectance andthe C/N of the GaBi film with the lapse of a long period of time;

FIG. 21 is a ternary phase diagram showing whether or not segregation iscaused when Se is added to the GaBi system;

FIG. 22 is a graph showing the change of the reflectance contrast,obtained when As is added to the GaBi film;

FIGS. 23 and 24 are graphs showing the change of reflectance and the C/Nof the InAs film with the lapse of a long period of time;

FIG. 25 is a ternary phase diagram showing whether or not segregation iscaused when Ge is added to the InAs system;

FIG. 26 is a graph showing the change of the reflectance contrast,obtained when Sn is added to the InAs film;

FIGS. 27 and 28 are graphs showing the changes of the reflectance andthe C/N of the InBi film with the lapse of a long period of time;

FIG. 29 is a graph showing the change of the reflectance contrast,obtained when Ga is added to the InBi film;

FIGS. 30 and 31 are graphs showing the changes of the reflectance andthe C/N of the GaSb film;

FIG. 32 is a ternary phase diagram showing whether or not segregation iscaused when Se is added to the GaSb system; and

FIG. 33 is a graph showing the change of the reflectance contrast,obtained when Te is added to the GaSb film.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An example of an information-storing optical system comprising thememory medium of the present invention is shown in FIG. 1. This systemis the same as the system used for the conventional write-once, holeablation type disk.

Light 2 (having ordinarily a wavelength of 780 to 830 nm) emitted from alaser diode 1 is passed through a shaping optical system 3, a polarizingbeam splitter 4, and a 1/4 wavelength plate 5, focussed by an objectlens 6, and applied to a memory film 7. In the drawings, referencenumerals 8 and 9 represent a substrate and a lens actuator,respectively. The reflected light is deflected in the transversedirection by the polarizing beam splitter 4 and guided to a lightdetector 11 through a lens 10. The light detector 11 is divided into 4parts, and the difference in signals of diagonal components indicatesthe degree of focus displacement.

Ordinarily, the laser diode 1 is driven by a direct current so that apower of about 1 mW is obtained on the surface of the memory film 7, andby using the reflected light from the memory film 7, the lens actuator 9for the object lens is controlled so that the light beams are alwaysfocussed on the film surface. The quantity of the reflected light fromthe memory film 7 is obtained as a sum signal of four parts of thedetector and used for determining the storage state of the signals, thatis, reproduction of information.

When information is recorded, a modulation current for modulating theintensity of the laser diode 1 is overlapped on the laser diode 1 by asignal to be recorded. When information is erased, the desired recordedportion is irradiated with continuous light beams. Also in this case, alight power necessary for erasure is overlapped on the light beam powerfor reproduction.

A stronger power is ordinarily more necessary at the recording step thanat the erasing step. It sometimes happens that erasure cannot becompleted by applying light beams only once. This is because a certaintime is necessary for changing the film to the erased state. In thiscase, if the same place is irradiated with erasing beams repeatedly (bya necessary number of rotations), a complete erasure state can beobtained.

Though not shown in FIG. 1, an optical system is often used in which twolaser beam sources are arranged, laser beams from one source are appliedthrough the same structure as shown in FIG. 1, and laser beams from theother source are applied in a shape long (up to about 10 μm) in thecircumferential direction on the film surface. In this case, the longbeams are used only for erasure, and information can be completelyerased by one irradiation.

The power conditions of light beams used for recording and erasiondepend on the diameter or rotation speed of the substrate, that is, thespeed of the memory film.

In the examples using an InSb memory film, shown hereinafter, the timeof irradiation of one point of the thin film with light beams having adiameter of 1 μm was changed by changing the rotation number and theradius point, and the relation between the power and irradiation time,showing optical changes corresponding to recording and erasion, wasdetermined to obtain data shown in FIG. 2. In FIG. 2, the ordinateindicates the irradiation beam power and the abscissa indicates theirradiation pulse time. Marks "∘" indicate an increase of thereflectance and marks "Δ" indicate a reduction of the reflectance. Itcan be seen that when strong short pulses are applied, the reflectanceof the film is increased, and when weak long pulses are applied, thereflectance is reduced.

As the reflectance is changed, so also is the transmittance changed. Inthe case of a film of InSb, when the reflectance is increased, thetransmittance is reduced, and when the reflectance is reduced, thetransmittance is increased. However, the change in the transmittance issmaller than that of the reflectance.

The amplitude of the signal is substantially proportional to thedifference of the reflectance between the recorded and erased states.The results obtained when the relative change of the amplitude of thesignal to the irradiation time for recording was determined are shown inFIG. 3, in which the ordinate indicates the relative contrast and theabscissa indicates the recording irradiation time. The power for therecording and the erasure conditions was fixed. As the irradiation timeis increased, the quantity of the relative change of the reflectance isincreased but, if the irradiation time exceeds a certain limit, therelative change quantity is reduced. Namely, optimum conditions arepresent.

Laser light, which is coherent light, is suitable as the recording andreproduction light. The wavelength is not limited to that ofsemiconductor laser light, but those of He-Ne laser light, He-Cd laserlight, and Ar laser light can be used.

As the result of analysis of the diffraction patterns shown in thephotos of FIGS. 4A and 5A, which were obtained in the after-mentionedexample 1, it was presumed that in the case of an InSb alloy film thecause of the change of the reflectance between two states of the crystalstructure would be as described below.

Only images (bright field images) shown in the transmission microscopephotos of FIGS. 4B and 5B corresponding to FIGS. 4A and 5A are examined.In the bright field images, the sizes of crystal grains in the centralportion are seemingly different, but the following has been confirmedfrom detailed analysis of the diffraction lines. In both FIGS. 4A and5A, In₅₀ Sb₅₀ (cubic crystal, a_(o) =6.478 Å) and Sb (hexagonal crystal,a_(o) =4.307 Å, c_(o) =11.273 Å) are observed, but the intensity ratioof the diffraction lines in FIG. 4A is the converse of that in FIG. 5A.Namely, in FIG. 4A, the diffraction line of In₅₀ Sb₅₀ is stronger thanthat of Sb, but in FIG. 5A, the diffraction line of Sb is stronger thanthat of In₅₀ Sb₅₀. This means that the amount of Sb precipitated fromthe alloy InSb is changed according to the irradiation conditions oflight. Since it is known that the reflectance of a pure Sb film is 70%and the reflectance of an In₅₀ Sb₅₀ film is 40%, it can be explainedthat the larger the amount of Sb precipitated, the higher thereflectance.

There are two probable causes of the manifestation of the difference inthe balance between In₅₀ Sb₅₀ and Sb. Namely, because of the differenceof the heating and cooling processes between two kinds of lightirradiations, (1) Sb element shifts transversely with respect to thefilm or (2) the amount of Sb solid-dissolved in In₅₀ Sb₅₀ is differentand the amount of Sb precipitated is different. At any rate, both thestates are apparently crystalline states.

Another cause of the formation of two crystalline states seeminglydifferent in reflectance in the film can be considered. For example,since the size of crystal grains are different, a difference of thecapacity of scattering light is brought about, which results in thedifference of the reflectance. In the above-mentioned example of InSb,the possibility that this mechanism makes a contributions to the changeof the reflectance cannot be denied.

Moreover, the change of the configuration of the film will result in thedifference in the manner of the light scattering. Apparently, the lightscattering effect obtained when the film surface is flat is differentfrom the light scattering effect obtained when the film is deformed tohave a concave or convex surface.

Still another possibility can be considered wherein even if the film iscrystalline, different crystal phases are formed according to thedifference of the cooling process. For example, when the film isirradiated with strong short light pulses, the film is molten, but sincethe film is abruptly cooled, there is a possibility of the formation ofa metastable crystal phase that cannot be obtained by the ordinarymelting, cooling, and solidification process.

As is apparent from the foregoing description, a film which iscrystalline and in which the reflectance or other optical characteristicis seemingly changed, irrespective of cause, may be changed, althoughvarious causes of this change can be considered.

We further investigated the facts as mentioned above and found that analloy composed of two or more elements capable of forming an eutecticmixture and having a composition close to an eutectic mixture in anequilibrium diagram of these two or more elements can be preferably usedfor the memory material in the invention, since it can form two stablecrystalline states differing in crystal texture and opticalcharacteristics when irradiated by optical energies under differentconditions.

The principle of the formation of two stable crystalline states due tothe eutectic phenomena is explained as follows.

FIG. 6 schematically shows a typical equilibrium diagram of an alloy oftwo elements.

The invention can be applied to not only this type of alloy having sucha simple equilibrium diagram but also to any alloys having anequilibrium diagram including an eutectic composition as a part thereof.Such alloys include Al-Ca, Al-Ge, Al-Mg, Al-Te, As-In, As-Pb, Au-Bi,Bi-In, Bi-Pb, Bi-Sn, Bi-Te, Ca-Mg, Ga-Sb, Ge-Sb, Ge-Te, In-Sb, In-Sn,In-Te, Mg-Sn, Pb-Sb, Pb-Sn, Pb-Te, Sb-Te, Sn-Te, Sn-Zn, Sn-Tl alloys andthe like.

In FIG. 6, we will consider a composition A close to a composition Z,although the composition Z in which pure elements X and Y are mixed inan appropriate ratio is an eutectic mixture or crystal. A film of analloy having the composition A usually can be formed by various methodssuch as vacuum-deposition and sputtering. The as-deposited film is anamorphous film in which the elements X and Y are mixed in an order ofatom or is composed of an assembly of extremely fine crystallites. Whena strong laser beam is irradiated onto the film at a diameter of about 1μm, by focusing the beam on the film to heat a small region of the film,that small region becomes a fused state corresponding to the point P inFIG. 6. After stopping the light irradiation, the temperature of thesmall region of the film gradually drops to the liquidus l₁. At thattime (point Q), deposition or crystallization of the element X begins.During cooling, as the heat is scattered and lost radially, a part ofthe small region which first reaches the point Q is the most outwardperipheral part of the fused region. As time passes and cooling proceedsfurther, the fused region is narrowed to a central portion of theoriginal fused region. Since the element X is gradually deposited at theperipheral portion, the concentration of the elements Y graduallyincreases in the average composition of the central fused portion, i.e.,that average composition moves toward the point R from the point Q onthe liquidus l₁. Finally reaching the eutectic temperature T₁, the fusedportion reaches the point R and is then crystallized to form an assemblyof crystallites of the elements X and Y having the eutectic compositionZ. This assembly of crystallites is not a mixture of the elements X andY in an order of atom, but is a mixture of crystallites of therespective elements X and Y, wherein the average composition thereof isZ.

However, it is supposed that as the fusion and solidification withirradiation of a laser beam are dynamic or occur in a nonequilibriumstate, the results do not always coincide with the equilibrium diagram.Because the fused region of the film formed by a laser beam is within anextremely small space as small as 1 μm or less in size, resulting insupercooling, rapid cooling etc. therein, even the assembly ofcrystallites does not become a mixture in which the crystallites X and Yare uniformly mixed. That is, as the crystallites of the element X havecrystallized at a peripheral portion of the original fused region,during solidification of a central portion, the crystallites of theelement X provide seeds for crystallization and, therefore, the elementsX tend to precedingly crystallize and the elements Y tend to remain in amore central portion. As a result, at a temperature of below T₁, adistribution of crystallites as shown in FIG. 7A results. In FIG. 7A, 21denotes an outside portion not irradiated with light and showing anamorphous or extremely fine crystallite state of a mixture of theelements X and Y, 22 a portion of crystalline element X, 23 a portion ofcrystalline element X including an admixture of crystallites of theelement Y, and 24 a central portion of crystalline element Y. This isone stable crystalline state of the memory film, which is used forrecording information. The feature of this state is that although theaverage element composition is A, the composition of a most centralportion is almost B which contains an extremely large amount of theelement Y.

The second stable crystalline state of the memory film is describedbelow. A laser beam is again irradiated to the memory film in the firststable crystalline state as described above. The laser beam usually hasan intension distribution in the form of the nomal distribution and,therefore, is strongest in its intensity at a central portion of thebeam. Thus a portion of the film which, by irradiation of the laserbeam, is heated to increase its temperatue and first reaches a fusedstate is a central portion of the beam-irradiated region. The fusedstate corresponds to the point L in the equilibrium diagram in FIG. 6since the central portion has a higher concentration of the element Ydue to the former irradiation. After stopping irradiation of the laserbeam, when cooling is proceeded and reaches the liquidus l₂, theelements Y begin to crystallize. However, since the peripheral portionof the fused region is surrounded by the crystalline element Y, there isno seed for crystallization of the element Y and, therefore, depositionof the element Y is not so rapid as in the former case at the point Q.Nevertheless, with the gradual proceeding of cooling, the composition ofthe fused portion approaches the eutectic point R and the composition ofthe central portion becomes the point A wherein the concentration of theelement X is higher as before when the fusion is finally solidfied. Thisstate is shown in FIG. 7B, which shows a feature wherein crystallites ofthe elements X and Y are mixed in a considerably disturbed state.

In FIG. 7B, 31 denotes an outside portion not irradiated with light andbeing in an amorphous or extremely fine crystallite state in which theelements X and Y are mixed, 32 a portion of crystalline element X, 33 acrystalline element Y comprising an admixture of crystallites of theelement X, and 34 a portion of a mixture of crystallites of the elementsX and Y where the element X is present in a larger amount than theelement Y.

The two states shown in FIGS. 7A dn 7B can be reversibly transformed.For example, the state in FIG. 7B can be transformed to the state inFIG. 7A by irradiating with a laser beam with a strong power for a shorttime, and in turn, the state in FIG. 7A can be transformed to the statein FIG. 7B by irradiating with a laser beam with a weak power for arelatively long time.

Usually, the crystalline states of the two kinds of elements, X and Y,have different optical characteristics. Therefore, if a central portionof a small region irradiated with a laser beam under differentconditions has a different amount of either one of the two elements Xand Y, that irradiated small crystalline portion has different opticalcharacteristics such as reflectance and transmittance. Thus, byirradiating the certain portion of the film with a weak laser beam anddetecting the intensity of the reflected or transmitted light, whichstate of the two states is present in the certain portion of the filmcan be recognized. As the two states can be reversibly transformed, byusing one state as a recorded state and the other state as an erasedstate, the film can be used as an erasable memory material in whichinformation can be freely recorded and erased.

In the invention, because the film necessarily uses the process offusion and solidification when one state storing information istransformed to the other erasure state, the transformation betweendifferent states is not limited to a particular time period, which isdifferent from the case of a, so-called, crystallization of an amorphousstate and allows a high speed recording and erasing.

EXAMPLE 1 Formation of InSb Film

Referring to FIG. 8, a film 42 of an alloy of In₄₀ Sb₆₀ was formed on anacrylic substrate 41 having an outer diameter of 30 cm and a thicknessof 1.2 mm, by vacuum deposition. The temperatures of the evaporationsources of the respective components were independently controlled andthe substrate was rotated so that the rates of evaporation of therespective components were constant. The thickness of the formed filmwas 90 nm. A protecting film 43 of an organic polymer having thicknessof 100 nm was formed on the alloy film. Ordinary the thickness of theprotecting film is 50 to 300 nm. Any material can be used for theprotecting film, in so far as it does not have a bad influence on theInSb recording film. For example, thermoplastic resins such as PMMA andpolystyrene (PS), and thermosetting resins such as an epoxy resin andultraviolet ray-curable resins may be used. As shown in FIG. 9, a verythin inorganic transparent film (for example, SiO₂, CeO₂, SnO₂ or ZnS)having a thickness of less than few hundred angstroms may be formed as astabilizing layer 44 between layers 41 and 42 and between layers 42 and43.

CHANGE OF REFLECTANCE

By using an optical head in which the beam diameter of semiconductorlaser light (λ=830 nm) was reduced to 1 μm by a collimator lens and anobject lens, the semiconductor laser light was directly modulated whilerotating the disk, and the disk was irradiated with the semiconductorlaser light. At this point, the position of the object lens wascontrolled so that the laser light was most concentrated on therecording film. The maximum of the intensity of the light beam appliedto the recording layer was 20 mW.

When the disk was irradiated with the laser light at a laser power of 5mW while rotating the disk at 600 rpm, the reflectance from therecording film was gradually reduced with the rotation. When the diskhad made 5 rotations, the change of reflectance was substantiallystopped, and hence, the laser power was reduced below 1 mW. Then, thesemiconductor laser was driven at a peak power of 20 mW by a rectangularwave of 2 MHz and the disk was irradiated with the laser light for onlyone rotation, whereby the reflectance of the portion exposed to thepulsative light was increased. When the reflectance on the disk wascontinuously measured at 1 mW, a signal of 2 MHz was detected at a C/Nof 40 to 45 dB.

When the disk was continuously irradiated at a power of 5 mW, thereflectance was reduced again and the signal component of 2 MHzdisappeared. Thus, it was confirmed that by irradiation with themodulated pulsatile light and continuous irradiation at a lower power,recording and erasure of signals could be repeated, and it was foundthat the repetition frequency exceeded 10⁴.

A part of the disk was separated, and the parted disk was irradiatedwith light pulses in the stationary state. Since the time required forone point of the recording flm on the disk in the rotating state totraverse the laser light (diameter φ=1 μm) was about 200 ns, irradiationwith the light pulse was effected in compliance with this time. Atfirst, when irradiation at a power of 5 mW for 200 ns was repeated 5times, the reflectance was reduced. Then, when the irradiation point waschanged and irradiation at 5 mW for 200 ns was repeated 5 times andirradiation at 20 mW for 200 ns was conducted once, the reflectance wasincreased. It was confirmed that, by repeating the above two operations,the reflectance was increased and reduced repeatedly.

Generally, a laser pulse having a power of 3 to 20 and a pulse width of50 to 200 ns may be used for recording, and a laser pulse having a powerof 1 to 8 mW and a pulse width of 0.1 to 10 μs may be used for erasing.

Evaluation of Crystal Structure

The recording film was peeled from the parted disk, and the crystalstructure of the film was examined by an electron microscope.

In the unrecorded portion which had not been irradiated with laser lightat all after formation of the film, diffraction of the electrons due toa regular arrangement of crystals was not observed and this portion wasin the amorphous state. In the portion where the reflectance was reducedby repeating irradiation with light pulses many times, as is seen fromthe photos of FIGS. 4A and 4B, complete crystallization of spots havinga diameter of about 1 μm was caused. When the portion where thereflectance was increased again by irradiation with strong pulses wasobserved, it was found that the crystalline state was similarly producedas is seen from the photos of FIGS. 4B dn 5B but the size of crystalgrains at the central part became larger. In FIGS. 4 and 5, the photosof FIGS. 4A and 5A show diffraction patterns by the electron microscopeand the photos of FIGS. 4B and 5B show bright field images by theelectron microscope. From this electron microscope observation, it wasconfirmed that information was not recorded by the phase transitionbetween the crystalline state and the amorphous state (the disturbedcrystalline state resembling the state just after formation of the film)but crystallization was once caused and information was recorded by thechange of the crystalline state.

Note, in the scanning electron microscope observation, it was found thatslight convexities and concavities were present in the irradiatedportion of the film, and it was confirmed that the direction ofconvexities and concavities in the recorded portion was the reverse ofthe direction of convexities and concavities in the erased portion.

Electric Conductivity

A film was formed on a quartz substrate in the same manner as describedabove and the film was heated in an electric furnace, taken out from thefurnace, and cooled at room temperature. Then, the electric conductivitywas measured. The obtained results are shown in the graph of FIG. 10. Itis seen that the electrical conductivity was abruptly increased at about190° C. This change of the electrical conductivity is due to thetransition to the crystalline state from the amorphous state. No greatchange of the electrical conductivity was observed above 200° C. Sinceit has been confirmed that both the recorded and erased states arecrystalline states from the results of the electron microscopeobservation, it is considered that there is no substantial difference ofthe electrical conductivity between the two crystalline states used forrecording information, contrary to the case of usingamnorphous-crystalline transition which is accompanied by a substantialchange of the electrical conductivity.

Durability Test

The above-mentioned information-recorded disk was placed in anatmosphere, maintained at a temperature of 70° C. and a relativehumidity of 85%, and at certain times, the temperature was lowered toroom temperature and the C/N ratio was measured. As shown in FIG. 11,even after the lapse of 3 months, the quantity of the reduction of theC/N ratio was smaller than 3 dB.

This indicates that the InSb film used for recording is chemicallystable and suitable for retaining information for a long time.

EXAMPLE 2 Composition Dependency of InSb Film

Films of alloys of In and Sb were prepared on acrylic substrates in thesame manner as adopted for formation of the film of In₄₀ Sb₆₀ in Example1, except that the composition was changed.

The so-prepared recording media were evaluated in the following manner.In the stationary state, the disk was irradiated alternately with twokinds of laser light pulses differing in power and pulse width by usingan optical head in which the beam diameter of a semiconductor laserlight (830 nm) was reduced to 1 μm by a collimator lens and an objectlens, and the reflectance was measured with the low power laser light.In some samples a difference was found between the reflectance afterirradiation with a laser light of 10 mW for 200 ns and the reflectanceafter irradiation with a laser light of 5 mW for 500 ns. The reflectancewas reversibly changed, and the reflectance was increased by large-powershort pulses and reduced by small-power short pulses. When thedependency of the reflectance on the composition of the alloy film wasexamined, it was found that the reflectance was reversibly changed ifthe Sb content was in the range of from 20 atom % or more (but not100%). However, in the region where the In content was high, segregationof In was caused and the film was not practically suitable. It was alsofound that, preferably, the Sb content is in the range of from 50 to 85atom %.

Equilibrium Diagram

FIG. 12 shows the equilibrium diagram of In-Sb binary alloy system (from"Constitution of binary alloy", MCGRAW-HILL). In an alloy of In and Sb,an intermetallic compound InSb is formed at an atom ratio of In:Sb of1:1 and an eutectic composition is taken at an atom ratio of In:Sb of31.7:68.3. From the equilibrium diagram in FIG. 12, theoretically, itcan be seen that an alloy composed of 50 atom % or less of In and 50atom % or more of Sb shows an eutectic phenomenon and thus is suitablefor a memory material, and particularly, an alloy composed of 15 to 50atom %, more preferably, 35 to 45 atom %, of In and 50 to 85 atom %,more preferably, 55 to 65 atom %, of Sb is a preferable memory materialsince if an alloy comprises more than 68.3 atom % of Sb, a considerablyhigh power is necessary for recording and erasing, or the sensitivity isreduced. These theoretical recognitions basically coincide with theexperimental results.

EXAMPLE 3

An additive was added to the medium of InSb and the effect was examined.Se was added in an amount of 5, 10 or 20 atom % based on the totalcomposition, and the resulting medium was evaluated according to themethod described in Example 2. By the addition of Se, the occurrence ofsegregation of In was prevented even if the In content was high. Asshown in FIG. 13, the region where segregation of In did not occur wasexpanded and it was found that the addition of Se is effective forstabilization. In FIG. 13, the hatched region is the region wheresegregation was not caused.

Similar results were obtained when Si, P, S, Ge or As was added insteadof Se.

EXAMPLE 4

Media were prepared by adding Zn in an amount of 5, 10 or 20 atom %based on the total composition while keeping the In/Sb atomic ratioconstant (40/60), and they were evaluated according to the methoddescribed in Example 2. The quantity of the change of the reflectancewas evaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Zn Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                25                                                           5                28                                                          10                33                                                          20                30                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased by theaddition of Zn.

Similar results were obtained when Al, Ag, Cd, Sn, Pb, Te or Bi wasadded instead of Zn.

EXAMPLE 5

Media were prepared by adding As in an amount of 5, 10 or 20 atom %based on the total composition while keeping the In/Sb atomic ratioconstant (40/60), and they were evaluated according to the methoddescribed in Example 2. The reflectance constants were determined bychanging only the power by high-power short-pulse laser light. Theobtained results are shown in FIG. 14. As is seen from the results shownin FIG. 14, the sensitivity of the recording medium was improved by theaddition of As.

Similar results were obtained when Ga, Pb or Sn was added instead of As.

EXAMPLE 6 TlBi Film

Films of alloys of Tl and Bi with various compositions were prepared onacrylic substrates in the same manner as in Examples 1 and 2 except thatthe thickness of the TlBi film was 80 nm.

The prepared recording media were evaluated in the same manner as inExample 2. In some samples a difference was found between thereflectance after irradiation with a laser light of 10 mW for 200 ns andthe reflectance after irradiation with a laser light of 5 mW for 1 μs.The reflectance was reversibly changed, and the reflectance wasincreased by large-power short pulses and reduced by small-power longpulses. When the dependency of the reflectance on the composition of thealloy film was examined, it was found that the reflectance wasreversibly changed if the Bi content was in the range of from 30 atom %or more (but not 100%). However, in the region where the Bi content wasin a range of 30 to 60 atom %, a change of characteristics with time wasremarkable and the film was not practically suitable. It was found that,preferably, the Bi content is in the range of from 60 to 90 atom %.

Electron microscope analysis and scanning microscope observation of thecrystal structure of the TlBi films show results similar to those ofExample 1.

To examine the durability of the TlBi medium, the TlBi medium wasdeposited on slide glasses and then heated at 200° C. for 30 minutes forcrystallization; the TlBi medium was not covered with a protectivelayer. A disc as shown in FIG. 8 was prepared with the TlBi medium andrecorded in the form of tracks at 600 rpm and 2 MHz. After the mediumwas retained at 70° C. and 85% RH, the changes of the reflectance of themedium on the slide glasses and the change of C/N of the disc weredetermined. The results are shown in FIGS. 15 and 16 respectively. Asseen in FIGS. 15 and 16, the change of the reflectance of the mediumwithout a protective layer was small even after 3 months and thereduction of C/N was below 3 dB.

EXAMPLE 7

An additive was added to the medium of TlBi and the effect was examined.As was added in an amount of 5, 10 or 20 atom % based on the totalcomposition, and the resulting medium was evaluated according to themethod described in Example 6.

It was found that, as a result of the addition of As, the change ofcharacteristics with time was reduced even in compositions having a highamount of Tl and, as seen in FIG. 17, the change with time of contrastof even a medium containing 50 atom % of Tl was reduced, making ituseful for stabilization.

Similar results were obtained when P, S, Se or Te was added instead ofAs.

EXAMPLE 8

Media were prepared by adding Zn in an amount of 5, 10 or 20 atom %based on the total composition while keeping the Tl/Bi atomic ratioconstant (30/70), and were evaluated according to the method describedin Example 6. The quantity of the change of the reflectance wasevaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Zn Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                18                                                           5                20                                                          10                23                                                          20                20                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased byaddition of Zn.

Similar results were obtained when Al, Si, Ge, Ag, Cd, Sn, Pb, Te, Sb orIn was added instead of Zn.

EXAMPLE 9

Media were prepared by adding Pb in an amount of 5, 10 or 15 atom %,based on the total composition while keeping the Tl/Bi atomic ratioconstant (30/70), and were evaluated according to the method describedin Example 6. The reflectance contrasts were determined by changing onlythe power by high-power short-pulse laser light. The obtained resultsare shown in FIG. 18. As is seen from the results shown in FIG. 18, thesensitivity of the recording medium was improved by the addition of Pb.

Similar results were obtained when In or Sn was added instead of Pb.

EXAMPLE 10 GaBi Film

Films of alloys of Ga and Bi with various compositions were prepared onacrylic substrates in the same manner as in Example 1 or 2.

The prepared recording media were evaluated in the same manner as inExample 2. In some samples a difference was found between thereflectance after irradiation with laser light of 15 mW for 200 ns andthe reflectance after irradiation with laser light of 5 mW for 1 μs. Thereflectance was reversibly changed, and the reflectance was increased bylarge-power short pulses and reduced by small-power long pulses. Whenthe dependency of the reflectance on the composition of the alloy filmwas examined, it was found that the reflectance was reversibly changedif the Bi content was in the range of from 15 atom % or more (but not100%). However, in the region where the Ga content was high, segregationof Ga was caused and the film was not practically suitable. It was foundthat, preferably, the Bi content is in the range of from 40 to 70 atom%.

Electron microscope analysis and scanning microscope observation of thecrystal structure of the BiGa films show results similar to those ofExample 1.

The durability test was carried out in the manner similar to that inExample 6, and similar results were obtained as shown in FIGS. 19 and20.

EXAMPLE 11

An additive was added to the medium of GaBi and the effect was examined.Se was added in an amount of 5, 10 or 20 atom % based on the totalcomposition, and the resulting medium was evaluated according to themethod described in Example 2. By the addition of Se, the occurrence ofsegregation of Ga was prevented even if the Ga content was high. Asshown in FIG. 21, the region where segregation of Ga did not occur wasexpanded and it was found that the addition of Se is effective forstabilization. In FIG. 21, marks "∘ " indicate non-occurrence ofsegregation and marks "x" indicate occurrence of segregation, and thehatched region is the region where segregation was not caused.

Similar results were obtained when Si, P, S, Ge or As was added insteadof Se.

EXAMPLE 12

Media were prepared by adding Zn in an amount of 5, 10 or 20 atom %based on the total composition while keeping the Ga/Bi atomic ratioconstant (60/40), and were evaluated according to the method describedin Example 2. The quantity of the change of the reflectance wasevaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Zn Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                20                                                           5                23                                                          10                23                                                          20                23                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased by theaddition of Zn.

Similar results were obtained when Al, Ag, Cd, Sn, Pb, Te, Sb or In wasadded instead of Zn.

EXAMPLE 13

Media were prepared by adding As in an amount of 5, 10 or 20 atom %based on the total composition while keeping the Ga/Bi atomic ratioconstant (60/40), and were evaluated according to the method describedin Example 2. The reflectance constants were determined by changing onlythe power by high-power short-pulse laser light. The obtained resultsare shown in FIG. 22. As is seen from the results shown in FIG. 22, thesensitivity of the recording medium was improved by the addition of As.

Similar results were obtained when In, Pb or Sn was added instead of As.

EXAMPLE 14 InAs Film

Films of alloys of In and As with various compositions were prepared onacrylic substrates in the same manner as in Example 2 except that thethickness of the InAs film was 100 nm.

The prepared recording media were evaluated in the same manner as inExample 2. In some samples a difference was found between thereflectance after irradiation with laser light of 20 mW for 200 ns andthe reflectance after irradiation with laser light of 5 mW for 500 ns.The reflectance was reversibly changed, and the reflectance wasincreased by large-power short pulses and reduced by small-power longpulses. When the dependency of the reflectance on the composition of thealloy film was examined, it was found that the reflectance wasreversibly changed if the As content was in the range of from 35 atom %or more (not 100 atom %). However, in the region where the As contentwas low, or the In content was high, segregation of In was caused andthe film was not practically suitable. It was found that, preferably,the As content is in the range of from 50 atom % or more (but not 100atom %).

Electron microscope analysis and scanning microscope observation of thecrystal structure of the InAs films show results similar to those ofExample 1.

The durability test was carried out in the manner similar to in Example6, with the following two kinds of samples.

Sample I:

An alloy of In in an amount of 20 atom % and As in an amount of 80 atom% was deposited on an acrylic substrate having a shape of a slide glassand then heat-treated at 80° C. for 2 hours for crystallization. Thissample had no protection layer.

Sample II:

While rotating sample I at 600 rpm, a focused beam of a semiconductorlaser was concentrically irradiated onto the alloy film to recordinformation of 2 MHz.

The results can be seen in FIGS. 23 and 24 and were similar to those ofExample 6.

EXAMPLE 15

An additive was added to the medium of InAs and the effect was examined.Ge was added in an amount of 5, 10 or 20 atom % based on the totalcomposition, and the resulting medium was evaluated according to themethod described in Example 2. By the addition of Ge, the occurrence ofsegregation of In was prevented even if the In content was high. Asshown in FIG. 25, the region where segregation of In did not occur wasexpanded and it was found that the addition of Ge is effective forstabilization. In FIG. 25, the hatched region is the region wheresegregation was not caused.

Similar results were obtained when Al, Si, Zn, Se or Te was addedinstead of Ge.

EXAMPLE 16

Media were prepared by adding Bi in an amount of 5, 10 or 20 atom %based on the total composition while keeping the In/As atomic ratioconstant (20/80), and were evaluated according to the method describedin Example 2. The quantity of the change of the reflectance wasevaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Bi Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                15                                                           5                20                                                          10                17                                                          20                14                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased by theaddition of Bi in an amount of 5 or 10 atom %.

Similar results were obtained when Pb, Tl, Ag, Sb or Cd was addedinstead of Bi.

EXAMPLE 17

Media was prepared by adding Sn in an amount of 5, 10 or 20 atom % basedon the total composition while keeping the In/As atomic ratio constant(20/80), and were evaluated according to the method described in Example2. The reflectance constants were determined by changing only the powerby high-power short-pulse laser light. The obtained results are shown inFIG. 26. As is seen from the results shown in FIG. 26, the sensitivityof the recording medium was improved by the addition of Sn.

Similar results were obtained when P, S, Ga, Pb or Si was added insteadof Sn.

EXAMPLE 17 InBi Film

Films of alloys of In and Bi with various compositions were prepared onacrylic substrates in the same manner as in Example 2 except that thethickness of the InBi films was 120 nm.

The prepared recording media were evaluated in the same manner as inExample 2. In some samples a difference was found between thereflectance after irradiation with laser light of 20 mW for 200 ns andthe reflectance after irradiation with laser light of 5 mW for 500 ns.The reflectance was reversibly changed, and the reflectance wasincreased by large-power short pulses and reduced by small-power longpulses. When the dependency of the reflectance on the composition of thealloy film was examined, it was found that the reflectance wasreversibly changed if the Bi content was in the range of from 10 to 40atom %. It was found that, preferably, the Bi content is in the range offrom 10 to 33%.

Electron microscope analysis and scanning microscope observation of thecrystal structure of the InBi films show results similar to those ofExample 1.

Durability tests were carried out in accordance with the procedures inExample 6, using the following three types of samples.

Sample I (reference):

An alloy of In in an amount of 70 atom % and Bi in an amount of 30 atom% was deposited on an acrylic substrate having a shape of a slide glass.This film was not heat-treated and was not covered with any protectivelayer.

Sample II (The Invention):

An alloy of In₇₀ Bi₃₀ was deposited on an acrylic substrate having ashape of a slide glass and was heat-treated at 80° C. for 2 hours forcrystallization. No protective layer was formed on the alloy layer.

Sample III (The Invention):

With rotating the Sample II at 600 rpm, a focused beam of asemiconductor laser is concentrically irradiated onto the medium torecord information of 2 MHz.

The results are shown in FIGS. 27 and 28.

The sample I demonstrated a remarkable change of the reflectance withtime, shown as the line I in FIG. 27. In contrast, sample II, an exampleof a medium according to the invention, demonstrated a very stablereflectance, shown as the line II in FIG. 27. In fact, the reflectanceof the sample hardly changed after 3 months. The change of thereflectance of sample III was similar to that of sample II.

The sample III demonstrated the change of C/N ratio, shown as the lineIII in FIG. 28. As seen in FIG. 28, sample III demonstrated only achange of C/N ratio of less than 3 dB after 3 months.

EXAMPLE 18

An additive was added to the medium of In₇₀ Bi₃₀ and the effect wasexamined. As was added in an amount of 5, 10 or 20 atom % based on thetotal composition, and the resulting medium was evaluated according tothe method described in Example 2. By the addition of As, the occurrenceof segregation of In was prevented even if the In content was high.

Similar results were obtained when Al, Si, Zn, Ge, Ag, Sb, Se or Te wasadded instead of As.

EXAMPLE 19

Media were prepared by adding Sn in an amount of 5, 10 or 20 atom %based on the total composition while keeping the In/Bi atomic ratioconstant (70/30), and were evaluated according to the method describedin Example 2. The quantity of the change of the reflectance wasevaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Sn Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                16                                                           5                21                                                          10                18                                                          20                15                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased by theaddition of Sn in an amount of 5 or 10 atom %.

Similar results were obtained when As, Cd, Tl or Pb was added instead ofSn.

EXAMPLE 20

Media were prepared by adding Ga in an amount of 5, 10 or 20 atom %based on the total composition while keeping the In/Bi atomic ratioconstant (70/30), and were evaluated according to the method describedin Example 2. The reflectance constants were determined by changing onlythe power by high-power short-pulse laser light. The obtained resultsare shown in FIG. 29. As is seen from the results shown in FIG. 29, thesensitivity of the recording medium was improved by the addition of Ga.

Similar results were obtained when P, S, Se or Sn was added instead ofAs.

EXAMPLE 21 GaSb Film

Films of alloys of Ga and Sb with various compositions were prepared onacrylic substrates in the same manner as in Example 2 except that thethickness of the GaSb films was 180 nm.

The prepared recording media were evaluated in the same manner as inExample 2. In some samples a difference was found between thereflectance after irradiation with laser light of 10 mW for 100 ns andthe reflectance after irradiation with laser light of 5 mW for 500 ns.The reflectance was reversibly changed, and the reflectance wasincreased by large-power short pulses and reduced by small-power longpulses. When the dependency of the reflectance on the composition of thealloy film was examined, it was found that the reflectance wasreversibly changed if the Ga content was in the range of from 60 atom %or less. However, in the region where the Ga content was more than 50atom %, striped patterns appeared on the film, which were assumed to becaused by a segregation of Ga, and the film was not practicallysuitable. In the region where the Ga content was less than 5 atom %,pile-up of the film occurred at the portion irradiated with a laserbeam, which was assumed to be caused by bubbles, and caused the level ofchange of the reflectance to become unstable, and thus the film was notsuitable in practical use. Thus, it was found that preferably, the Gacontent is in the range of from 5 to 50 atom %.

Electron microscope analysis and scanning microscope observation of thecrystal structure of the GaSb films show results similar to those inExample 1.

The durability tests were carried out in the same manner as in Example17, using an alloy of Ga in an amount of 35 atom % and Sb in an amountof 65 atom %.

The results are shown in FIGS. 30 and 31 and are similar to those inExample 17.

EXAMPLE 22

An additive was added to the medium of Ga₃₅ Sb₆₅ and the effect wasexamined. Se was added in an amount of 5, 10 or 15 atom % based on thetotal composition, and the resulting medium was evaluated according tothe method described in Example 2. By addition of Se, occurrence ofsegregation of Ga was prevented even if the Ga content was high. Asshown in FIG. 32, the region where segregation of Ga did not occur wasexpanded and it was found that the addition of Se is effective forstabilization. In FIG. 32, the hatched region is the region wheresegregation was not caused.

Similar results were obtained when Al, Si, Zn, Ge or As was addedinstead of Se.

EXAMPLE 23

Media were prepared by adding Sn in an amount of 5, 10 or 20 atom %based on the total composition while keeping the Ga/Sb atomic ratioconstant (35/65), and they were evaluated according to the methoddescribed in Example 2. The quantity of the change of the reflectancewas evaluated based on the reflectance contrast obtained by dividing thequantity of the change of the reflectance by the reflectance of thehigh-reflectance state. The obtained results are shown below.

    ______________________________________                                        Sn Content (atom %)                                                                             Contrast (%)                                                ______________________________________                                         0                19                                                           5                24                                                          10                29                                                          20                26                                                          ______________________________________                                    

From the above results, it is seen that the contrast is increased by theaddition of Sn.

Similar results were obtained when As, Pb, or Zn was added instead ofSn.

EXAMPLE 24

Media were prepared by adding Te in an amount of 5, 10 or 20 atom %based on the total composition while keeping the Ga/Sb atomic ratioconstant (35/65), and were evaluated according to the method describedin Example 2. The reflectance constants were determined by changing onlythe power by high-power short-pulse laser light. The obtained resultsare shown in FIG. 33. As is seen from the results shown in FIG. 33, thesensitivity of the recording medium was improved by the addition of As.

Similar results were obtained when In, P, S, Se or Sn was added insteadof Te.

EXAMPLE 25 SnPb Film

A film of an alloy of Sn and Pb was prepared on an acrylic substrate inthe manner as in Example 1, with the average composition of the alloybeing 85 atom % of Sn and 15 atom % of Pb. The Sn-Pb alloy system has aneutectic composition where the Pb content is 26.1 atom % and the meltingpoint is 183° C.

When the C/N ratio of the SnPb film was measured with the optical systemin FIG. 1, as in Example 1, a C/N of 35 dB was obtained. In a repetitiontest of recording and erasing, the C/N did not change up to 100repetitions. However, the C/N was reduced by 5 dB one day afterrecording and it was found that the film has a slight durabilityproblem.

EXAMPLE 26 SnTe Film

A film of an alloy of Sn and Te was prepared on an acrylic substrate inthe same manner as in Example 1, with the average composition of thealloy being 30 atom % of Sn and 70 atom % of Te. The Sn-Te alloy systemhas an eutectic composition where the Te content is 84 atom % and themelting point is 405° C.

C/N ratio was measured and a C/N of 45 dB was obtained. The reflectanceof this film was increased when irradiated with a low-power long-pulselaser beam and decreased when irradiated with a high-power short-pulselaser beam.

We claim:
 1. An optical information memory medium including a substrate,comprising:a thin memory film, formed on the substrate including notmore than 60 atom % of Gallium (Ga) and not less than 40 atom % ofAntimony (Sb), capable of selectively forming two stable crystallinestates, said memory film having a first crystalline state wheninformation has been recorded and a second crystalline state wheninformation has been erased, the first crystalline state having a firstreflectivity by irradiating said memory film with an optical energy beamhaving a first intensity for a first time period such that the entirethickness of said memory film is fused at the irradiated portion, andthe second crystalline state having a second reflectivity lower than thefirst reflectivity by irradiating said memory film with an opticalenergy beam having a second intensity less than or equal to the firstintensity for a second time period longer than the first time period. 2.An optical information memory medium according to claim 1, wherein saidmemory film comprises 5 to 50 atom % of Ga and 50 to 95 atom % of Sb. 3.An optical information memory medium according to claim 1, wherein saidmemory film comprises 15 to 35 atom % of Ga and 65 to 85 atom % of Sb.4. An optical information memory medium according to claim 1, whereinsaid memory film further comprises at least one element M selected fromthe group consisting of Al, Si, P, S, Zn, Ge, As, Se, In, Sn, Te, Bi andPb, and wherein said memory film has a composition of (Ga_(x)Sb_(1-x))_(1-y) My where x≦0.60 and y≦0.20.